Conventional articles of athletic footwear include two primary elements, an upper and a sole structure. The upper is often formed of leather, synthetic materials, or a combination thereof and comfortably secures the footwear to the foot, while providing ventilation and protection from the elements. The sole structure generally incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin cushioning member located within the upper and adjacent the sole of the foot to enhance footwear comfort. The midsole, which is traditionally attached to the upper along the entire length of the upper, forms the middle layer of the sole structure and serves a variety of purposes that include controlling potentially harmful foot motions, such as over pronation, attenuating ground reaction forces, and absorbing energy. In order to achieve these purposes, the midsole may have a variety of configurations, as discussed in greater detail below. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear resistant material that includes texturing to improve traction.
The primary element of a conventional midsole is a resilient, polymer foam material, such as polyurethane or ethyl vinyl acetate, that extends throughout the length of the footwear. The properties of the polymer foam material in the midsole are primarily dependent upon factors that include the dimensional configuration of the midsole and the specific characteristics of the material selected for the polymer foam, including the density of the polymer form material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and energy absorption properties may be altered to meet the specific demands of the activity for which the footwear is intended to be used.
In addition to polymer foam materials, conventional midsoles may include, for example, stabilizing devices that resist over-pronation and moderators that distribute ground reaction forces. The use of polymer foam materials in athletic footwear midsoles, while providing protection against ground reaction forces, may introduce instability that contributes to a tendency for over-pronation. Pronation is the inward roll of the foot while in contact with the ground. Although pronation is normal, it may be a potential source of foot and leg injury, particularly if it is excessive. Stability devices are often incorporated into the polymer foam material of the midsoles to control the degree of pronation in the foot. Examples of stability devices are found in U.S. Pat. No. 4,255,877 to Bowerman; U.S. Pat. No. 4,287,675 to Norton et al.; U.S. Pat. No. 4,288,929 to Norton et al.; U.S. Pat. No. 4,354,318 to Frederick et al.; U.S. Pat. No. 4,364,188 to Turner et al.; U.S. Pat. No. 4,364,189 to Bates; and U.S. Pat. No. 5,247,742 to Kilgore et al. In addition to stability devices, conventional midsoles may include fluid-filled bladders, as disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Marion F. Rudy, for example.
As an alternative to the conventional midsole structures discussed above, various articles of footwear include flexible plates within the sole structure. For example, U.S. Pat. No. 4,566,206 to Weber discloses an article of footwear having a sole structure that includes a lower plate, an intermediate plate, and an upper plate. The lower plate extends along the entire longitudinal length of the footwear, whereas the upper and intermediate plates are positioned within a heel area of the sole structure. The upper and intermediate plates are joined with the lower plate and extend upwardly and rearwardly from, and at acute angles with respect to, the lower plate. In operation, the upper and intermediate plates provide different spring rates upon compression of the heel area of the sole structure.
U.S. Pat. No. 5,367,790 to Gamow et al. also discloses an article of footwear with a sole structure that includes flexible plates. The sole structure includes an upper plate that extends along the longitudinal length of the footwear and is secured to the upper. In addition, the sole structure includes a lower plate that is joined with the upper plate approximately two-thirds of the distance from the rear of the footwear to the front of the footwear. The upper plate and the lower plate purportedly form a collapsible longitudinal arch that stores energy during compression. A similar configuration is disclosed in U.S. Pat. No. 5,701,686 to Herr et al., which also discloses an additional forefoot plate that provides a spring structure in a forefoot portion of the footwear.
It is an object of the present invention to provide an article of footwear with a heel plate assembly that reduces or overcomes some or all of the difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.